Crowded houses and apartments are often cited as a source of spread, particularly in Los Angeles, which has some of the densest neighborhoods in the US Households in and around LA often have several generations — or multiple families — living under one roof. Those tend to be lower-income areas where residents work essential jobs that can expose them to the virus at work or while commuting.
This is exactly the pattern in NYC as well. High income areas, where 2-3 people live together in an apartment, such as LIC, Greenpoint and most of Manhattan, have low spread. A lot of people living together in a small space is going to encourage spread. When Cuomo talks of this being a virus of density, he means the city as a whole. But it's the indoor density that counts.
But also remember that for decades the city's position was that if you push for lower zoning in neighborhoods where illegal conversions are rampant, or demand enforcement of existing occupancy codes, you are a racist. We have groups now pushing for legalizing cellar and attic apartments, which in many cases have no ventilation. This isn't going to stop the spread, but rather encourage it.
Remember this as the dope-a-crats chide people over wearing masks and wanting to eat indoors. This entire pandemic response is a classic case of tweeding in lieu of actually governing effectively.